The invention concerns a filter system, in particular as part of a fuel vapor buffer installation for reducing hydrocarbon emission.
The active carbon filter systems or fuel vapor buffer installations, referred to as active carbon filters among experts, that are incorporated in vehicles and designed to prevent that vapors from the fuel tank are released into the environment, contain active carbon granules and regularly have to be rinsed or regenerated with fresh air. The corresponding rinsing cycles and rinsing volumes, however, are gradually decreasing so that, as a consequence, hydrocarbons that cannot be completely removed as a result of the feasible rinsing processes remain bonded in the active carbon filter. For testing the emissions escaping from a vehicle and thus from the active carbon filter, first of all the vehicle is subjected to a certain driving cycle. During such driving cycle the rinsing processes take place, which, however, are restricted to certain operational conditions of the vehicle and, consequently, do not suffice any more for sufficiently rinsing the active carbon filter. As a consequence, the hydrocarbons may remain bonded in the active carbon filter. After conclusion of the driving cycle, the vehicles are immediately taken into a so-called SHED chamber and subjected to a test lasting up to three days. Within the scope of the test, the vehicle is not moved, however it is subjected to broad-scale temperature variations. Accordingly, during the SHED test, hydrocarbon emissions escape from the vehicle in its entirety which also concerns fuel vapors from the tank system including the active carbon filter. It has turned out that the problem during the SHED test is not only the evaporation from the tank but also the evaporation of detrimental hydrocarbons remaining in the active carbon filter. In respect to vehicles launched on the US market threshold values for admissible hydrocarbon emission are particularly low so that on this market, in addition to the active carbon granules, a separate or additional filter system has to be incorporated into the fuel vapor buffer installation.
What is disadvantageous about current additional filter systems of the active carbon filter or the fuel vapor buffer installation is the rinsing process. The active carbon filter and thus also the additional filter system as part of the active carbon filter are arranged in the ventilation line of the tank. In the case of tank refueling or a temperature-induced extension of the contents of the tank, the air present in the tank has to be dissipated through the active carbon filter. Consequently, the active carbon filter is designed to prevent that fuel vapors are released into the atmosphere. However, in the case of active carbon filters it is required that intermittently a so-called rinsing process is continuously carried out. Thus, atmospheric air is reversely guided through the active carbon filter in order to regenerate the fuel vapor buffer installation, i.e. to rinse out the hydrocarbon vapors bonded in the active carbon granules and in the additional filter system. Herein, also fuel vapors that are not bonded in the active carbon granules or in the filter of the additional filter system but in peripheral components, such as foams or seals used for fixation or sealing, are rinsed. After passing the fuel vapor buffer installation and having been enriched with fuel vapors, the rinsing air is fed to the combustion process in the motor.
In the active carbon filters according to prior art having additional filter systems, sealing elements are provided, wherein it has been found that the rinsing processes meanwhile do not suffice any more to completely clean the active carbon filter due to the fact that an efficient cleaning of the peripheral components, such as the foam circle, the foam strip or the sealing elements, may only be effected completely provided that the rinsing period is very long, which, however, is not possible in practical operation.